A Ni-based superalloy which includes many alloy elements such as Al and Ti and is a γ′ (gamma prime) phase-precipitation strengthened type is used as a heat resistant member for aircraft engines and gas turbines for power generation.
A Ni-based forged alloy has been used as a turbine disk which requires high strength and reliability among components of a turbine. Here, the forged alloy is a term used in contrast to a cast alloy having a cast solidification structure which is used itself. The forged alloy is a material produced through a process in which an ingot obtained by melting and solidification is subjected to hot working and thereby a predetermined component shaped is made. Since hot working causes a cast solidification structure which is coarse and heterogeneous to be changed to a forged structure which is fine and homogeneous, mechanical characteristics such as tensile characteristics or fatigue characteristics are improved. For engine members for an aircraft and a gas turbine member for power generation, the temperature exposed and the degree of stress loaded during an operation of a turbine is deferent among the members. Thus, it is necessary that the balance between yield strength, fatigue strength, and creep strength of a material is optimized in accordance with a load status of each of the members. Generally, when the balance is optimized, it is important to allow a control of a grain size of a γ (gamma) phase forming a matrix in a Ni-based superalloy, in accordance with the purpose of a use. In order to improve yield strength or fatigue strength, it is important to reduce the grain size of grains in the matrix. However, as the size of materials of a product is increased, it becomes much more difficult to strictly control the grain size.
In order to improve engine efficiency, it is effective that a turbine is operated at an extremely high temperature. For this, it is necessary that a durable temperature of each turbine member is set to be high. In order to increase the durable temperature of a Ni-based superalloy, it is effective that the amount of the γ′ phase is increased. Thus, an alloy having a large amount of the precipitated γ′ phase is used in a member requiring high strength, among forged alloys. The γ′ phase corresponds to an intermetallic compound including Ni3Al. The material strength is increased more by dissolving elements which are represented by Ti, Nb, and Ta, in the γ′ phase. However, if the amount of Al, Ti, Nb, or Ta which is a constituent element of such a γ′ phase is increased, the amount of the γ′ phase which is a strengthening phase becomes excessive, and thus, it is difficult to perform hot working represented by press forging and the excessive amount of the γ′ phase causes a crack to occur in a hot working material in production. Thus, a component such as Al or Ti, which contributes to strengthening is generally limited in comparison to a cast alloy which is obtained without hot working. As a turbine disk material having strongest a strength currently, Udimet720Li (Udimet® is a registered trademark of Special Metals Co., Ltd.) is exemplified. In mass %, the amount of Al is 2.5% and the amount of Ti is 5.0%. The amount of the γ′ phase is about 45% at 760° C. Since Udimet720Li has a high strength and has a large amount of the γ′ phase, Udimet720Li is one of Ni-based superalloys on which performing hot working is most difficult.
As described above, regarding the forged alloy used in a turbine disk, a big challenge for a material is to achieve both strength and hot workability, and an alloy component for solving this challenge and a producing method thereof are researched.
For example, Patent Document 1 discloses the invention of a high-strength alloy which can be produced by a melting and forging process in the related art. In comparison to Udimet720Li, the alloy includes a lot of Ti and has a high structural stability by adding a lot of Co, and hot working is also possible. However, this alloy also has the amount of the γ′ phase which is 45% to 50%, that is, large similarly to that in Udimet720Li. Thus, hot working is very difficult.
There is an attempt to improve hot workability by a production process. In Patent Document 1, regarding a forged article of Udimet720Li, an experiment result in that hot workability is improved as a cooling rate after the temperature is increased to 1110° C. becomes slower is disclosed. Although improvement of hot workability by a heat treatment is an important knowledge, in a practical hot-working process, after a hot working material is drawn out from a heating furnace, a surface temperature of the hot working material is significantly decreased by a contact with an outside air or a die of a hot working device. At this time, a problem remains in that the γ′ phase is precipitated in the process of cooling the surface of the material, and the precipitated γ′ phase causes deformation resistance to be increased and causes a hot working crack in the surface.
In a case where a Ni-based superalloy which has a large amount of the γ′ phase constituent element such as Al and Ti is subjected to hot working, the followings are known. The γ′ phase is precipitated by decreasing the temperature of the material during the hot working. Thus, hot workability of the hot working material is significantly degraded and a crack often occurs in the hot working material by the working. Therefore, in a case where it is assumed that such a Ni-based superalloy is subjected to hot working, various attempts for suppressing the decrease of the temperature of the material during the hot working are made.
For example, a method in which working is ended before the temperature of the material is decreased, by increasing a working speed, or a method in which the working amount for one time is reduced and hot working is performed by performing reheating plural number of times is considered. If the working speed is increased as in the former case, modification of a microstructure by working heat generation, that is, coarsening of crystal grains of a γ matrix phase or incipient melting at a grain boundary of the matrix easily occurs. In the latter case, there are problems in that the amount of hot working for one time is necessarily small and energy required for production is increased, and that, since non-uniform deformation by hot working plural number of times easily occurs, it is difficult to obtain a desired product shape, and that homogeneity of the microstructure is easily lost.